Without limitation, the background is described in connection with diodes and integrated circuit structures for Electrostatic discharge (ESD) protection. ESD protection for integrated circuits is vital to protect them from degradation and damage due to contact with manufacturing machines, personnel, and users and/or with any objects that can convey static electricity into the integrated circuits. A sudden spark or surge of static electricity into an integrated circuit is called a strike event. Integrated circuits have diodes and transistors among other possible structures deposited and formed therein according to various processes.
A diode has a junction with junction corners or a silicon/oxide interface or an interface with a junction having carrier depletion in some other materials system. A diode conducts well in one direction of electric current flow and poorly or negligibly in an opposite direction. An electric field is the property by an electric force is provided to cause more or less current flow depending on the conducting ability of a semiconductor structure. Electric potential is that property of electricity represented by spatial contour lines (electric equipotential lines of equal voltage) wherein nearer closeness of the contour lines represents a higher intensity electric field. Electric current flows, if at all, in a direction perpendicular to the contour lines like water flows down a hill perpendicular to the contour lines of the hill, i.e. lines joining points of equal elevation.
In an ESD strike event, a conventional diode fails at the junction corner or silicon/oxide interface due to the larger or more acute curvature of electric equipotential lines, compressing the electric equipotential lines closer together and resulting in higher electric field and current flow in the regions.
FIG. 1 illustrates a microscopic cross-section of a conventional ESD diode structure. An NWELL defines PSD/NWELL junction breakdown voltage. NBL & DeepN minimize resistance in current path. (For the acronyms, see Glossary TABLE 1.)
ESD diodes should not conduct more than negligibly when the integrated circuit being protected is being powered with ordinary operating voltage. However, ESD diodes should conduct and not fail when much greater than ordinary operating voltage at same polarity is impressed on the integrated circuit, or when any voltage of opposite polarity is undesirably present. Or in substantial ESD strikes, indeed, ESD diodes should conduct as much current as possible to minimize the chance of the devices failing at such ESD strikes.
FIG. 2 depicts the equipotential lines at the 0.7th nanosecond of a standardized electrostatic discharge pulse called an IEC pulse. In FIG. 3, when an ESD diode is considered microscopically, the corresponding electric field is caused to be substantially increased at the junction corner due to the spatial curvature at the corner of the electric potential of FIG. 2. That curvature brings the potential lines there much nearer one another and causes a higher electric field in only one or more of such relatively small junction corner portions of the larger diode structure. Thus, the potential curvatures result in higher electric field at the edge/corner of a conventional junction.
In FIG. 4, the higher electric field also triggers more current flowing through the edge or corner of the junction (avalanche) than other parts of the diode. This very substantial non-uniformity of electric field inefficiently concentrates current flow in the diode, when it occurs, into a very small part of the diode structure, and thus poorly uses the diode structure for ESD purposes. In a reversed bias with high current operation in such a diode at an ESD strike, the enhanced electric field at a junction corner conducts more current (avalanche). In FIG. 5, the higher current heats the diode to high temperatures at such corner and causes junction failures at the corner. The premature failure degrades the ESD performance of IC devices. Thus, the higher electric field at the corner leads to higher current at the corners, which leads to higher temperatures there. An ESD diode can fail prematurely at the corner due to the high electric field itself or due to thermal runaway degrading the properties of the semiconductor material there and allowing yet more electric current to flow and thereby causes the diode to fail prematurely.
Accordingly, significant departures and alternatives in circuits, structures, and processes for addressing the above considerations and problems would be most desirable.